1. Field of the Invention
The present invention relates to a honeycomb-shaped substrate, which is used for catalysts for purifying automotive exhaust gases, and a catalyst for purifying exhaust gases, catalyst which uses the honeycomb-shaped substrate.
2. Description of the Related Art
In catalysts for purifying exhaust gases, such as three-way catalysts, which have been employed in automobiles, a highly heat resistant honeycomb-shaped substrate, which is made from ceramic, such as cordierite, has been used. The honeycomb-shaped substrate comprises cellular walls, and a large number of cellular passages, which are demarcated by the cellular walls. On a surface of the cellular walls, a catalytic coating layer is formed. The catalytic coating layer is made by loading a catalytic ingredient, such as platinum, on a porous support powder, such as a γ-alumina powder, and thereby the honeycomb-shaped substrate is used as a catalyst for purifying exhaust gases.
The resulting catalyst for purifying exhaust gases produces a great contacting area between the catalytic ingredient and exhaust gases, because exhaust gases distribute through the cellular passages whose peripheral walls are provided with the catalytic coating layer. Thus, the exhaust-gases-purifying catalyst exhibits extremely good purifying conversions.
In order to form the catalytic coating layer, the following method has been employed in general. An oxide support powder is turned into a slurry along with a binder and water. After filling the resultant slurry into the cellular walls of the honeycomb-shaped substrate, the excessive slurry is discharged out of the honeycomb-shaped substrate, and thereafter the honeycomb-shaped substrate is calcined. Then, a catalytic ingredient is loaded on the thus formed coating layer. Recently, however, such a method has become commonplace that a slurry is prepared using a catalytic powder, which is made by loading a catalytic ingredient on an oxide support powder in advance; and then the resulting slurry is coated on a honeycomb-shaped substrate.
However, by means of the above-described so-called wash coating method, a coating layer which has a uniform thickness has been formed on a honeycomb-shaped substrate entirely. When a catalytic coating layer with a uniform thickness is formed on a honeycomb-shaped substrate, the temperature of the honeycomb-shaped substrate's exhaust-gas-flow upstream portion has become high temperatures locally in service as a catalyst for purifying exhaust gases. Accordingly, the resultant exhaust-gases-purifying catalyst might be associated with a problem that the durability has deteriorated considerably.
Namely, a catalyst for purifying exhaust gases is likely to exhibit high temperatures on the inlet side into which high-temperature exhaust gases flow. Moreover, since the reaction heat, which results from reactions on the inlet side, propagates to the exhaust-gases-purifying catalyst's exhaust-gas-flow downstream side, reactions, which are similar to the inlet-side reactions, take place on the exhaust-gas-flow downstream side as well. The phenomenon is advantageous in view of the purifying performance in a low-temperature region. However, in a temperature region where the temperatures are higher than a catalytic ingredient's activation temperature, the reaction heat, which derives from the reaction on the exhaust-gas-flow upstream side, becomes less likely to dissipate. Consequently, the catalytic ingredient has undergone a granular growth, and the like, especially on the exhaust-gas-flow upstream side so that the exhaust-gases-purifying catalyst might exhibit considerably deteriorated durability.
In order to inhibit the above-described problem, it is possible to make the loading amount of a catalytic ingredient less on the exhaust-gas-flow upstream side of the exhaust-gases-purifying catalyst and make it more on the exhaust-gas-flow downstream side thereof. If such is the case, however, the resultant exhaust-gases-purifying catalyst might have such a problem that the ignitability has degraded and the low-temperature purifying performance has degraded because it exhibits degraded reaction activities on the exhaust-gas-flow upstream side. Moreover, by means of the wash coating method in which a slurry, which is made by using a catalytic powder with a catalytic ingredient loaded on an oxide support powder in advance, is wash coated onto a honeycomb-shaped substrate, it is difficult to make the loading amount of the catalytic ingredient on the exhaust-gas-flow upstream side different from that on the exhaust-gas-flow downstream side, because the resulting coating layer is formed in a uniform thickness so that the catalytic ingredient has been loaded in a uniform loading amount.
Moreover, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-838 discloses a catalyst for purifying exhaust gases, catalyst which comprises an upstream-side coating layer and a downstream-side coating layer. The upstream-side coating layer includes Al2O3 whose pore volume is 0.6 c.c./g or less, and is formed on an exhaust-gas-flow upstream portion of the catalyst. The downstream-side coating layer includes Al2O3 whose pore volume is 0.8 c.c./g or less, and is formed on an exhaust-gas-flow downstream portion of the catalyst.
The conventional catalyst disclosed in the gazette exhibits a high thermal conductivity, because the upstream-side coating layer has a dense structure. Moreover, the heat of exhaust gases are transmitted quickly to the downstream-side coating layer, because the densified upstream-side coating layer is disposed on an exhaust-gas-flow upstream side of the catalyst. Accordingly, the conventional catalyst can purify hydrocarbons (hereinafter abbreviated to “HCs”) even from a low-temperature region, because it is good in terms of the temperature increment characteristic. In addition, the downstream-side coating layer in which Pd is loaded on Al2O3 whose pore volume is 0.8 c.c./g or less is good in terms of the gas diffusability so that HCs diffuse quickly, because it exhibits a large pore volume. Consequently, the conventional catalyst shows good conversions.
Such a coating operation as coating the upstream-side coating layer and downstream-side coating layer dividedly can be carried out as follows. First of all, a first slurry is filled into a honeycomb-shaped substrate by a predetermined depth from one of the honeycomb-shaped substrate's opposite ends on down. The excessive slurry is removed by suction through the one of the honeycomb-shaped substrate's opposite ends, thereby forming a first coating layer. Then, the resulting first coating layer is dried and/or calcined. Subsequently, a second slurry is filled into the honeycomb-shaped substrate by another predetermined depth from the other one of the honeycomb-shaped substrate's opposite ends on down. The excessive slurry is removed by suction through the other one of the honeycomb-shaped substrate's opposite ends, thereby forming a second coating layer. Then, the resulting second coating layer is dried and/or calcined. Moreover, in this method, it is possible to make the thickness of the upstream-side coating layer different from that of the downstream-side coating layer by making the viscosity of the first slurry different from that of the second slurry, for instance.
However, in the above-described method, it is needed to repeatedly carry out the coating step and the drying-and/or-calcining step several times. The method might have a problem in that the man-hour requirements have been excessive.
In addition, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2003-225,540 discloses an apparatus for purifying exhaust gases, apparatus which comprises a diesel particulate filter (hereinafter abbreviated to as “DPF”) on which a catalytic coating layer is formed. The DPF exhibits a larger average pore diameter on the exhaust-gas-flow upstream side, and exhibits a smaller average pore diameter on the exhaust-gas-flow downstream side. However, the DPF is formed as wall-flow structure, and has cellular walls whose pore diameters are about some dozens of micrometers in order to collect particulate matters in exhaust gases. Therefore, it is not possible to establish the present invention using a honeycomb-shaped substrate with such large pores.